By one procedure in the prior art, the amount of wetting agent contained in an acidic tin-lead bath is determined, for example, by taking 10.0 ml of the bath fluid, mixing it with about 80.0 ml of distilled water, and adding 20.0 ml of dilute nitric acid. The diluted and acidified bath specimen is slowly titrated by means of a tannic acid solution until opalescence remains. The tannic acid solution used for titration contains about 1 gm of tannic acid per liter of distilled water. The nitric acid serving as mixing agent for the diluted bath fluid is thinned to a ratio of about 1:5. When titrating the bath specimen and when determining the quantitative level of surfactant or wetting agent in the bath specimen, the amount of tannic acid solution consumed up to the point where opalescence remains (persists) is measured. However, the effective component of the tannic acid solution is not constant which makes it necessary to carry out a second titration with the same bath fluid and which is accomplished adding a measured amount of a standard surfactant solution. The quantitative level of surfactant in the bath specimen is established by calculation from the two values measured for the amount of tannic acid solution used. To determine the titration point, use is made of a base which is as black as possible and which serves as a contrasting background to confirm when opalescence remains.
This known procedure described above has proven its worth for acid tin-lead baths; however, it is not suitable for automatically establishing the level of nonionic surfactant in acid tinning baths because of the fact that the bath fluid in tinning baths exhibits a comparatively high degree of natural cloudiness and becomes even cloudier when dilute nitric acid is added before titration even starts. Precise recognition of the final point is consequently precluded. In addition, a visual recognition of the final titration point cannot be mechanised. It must be merely assumed that, during titration, the commencement of the clouding process is influenced by other substances contained in the bath fluid in the bath to be tested. Consequently, the final or titration point cannot be determined with the sufficient precision required in precise automated processing.